Compression method and air separation

ABSTRACT

A compression method, a multistage compression system incorporating such method and an air separation method and plant utilizing such compression method and system in which a gas is compressed in a series of compression stages to produce a compressed gas and each of the compression stages incorporate a variable speed compressor. In such compression method and system, the compressed air is produced at a pressure that remains stable during both normal operational conditions and during turn down conditions during which the flow rate of the gas is reduced. This reduction is accomplished by reducing the speed of the compressor in an initial compression stage such that the compressor operates along a peak efficiency operating line at which the pressure ratio is directly proportional to the flow rate and such that the lower turn down flow rate is obtained at a reduced pressure which is made up in successive compression stages.

FIELD OF THE INVENTION

The present invention relates to a method and system for compressing a gas and an air separation method and plant incorporating such method and system in which a gas or air is compressed in a series of compression stages to a higher pressure that is maintained both at normal operating conditions and during turndown operational conditions when a lower flow rate of the gas or air is required. More particularly, the present invention relates to such a method and system and air separation method and plant in which the compression stages have variable speed motors driving compressors and the speed of each of the compressors is adjusted during turndown such that an initial of the compression stages operates along a peak efficiency operating line.

BACKGROUND OF THE INVENTION

Many industrial processes require the compression of a gas. For example, in air separation, air is compressed, cooled to a temperature at or near the dew point and then introduced into a distillation column system to separate the air by cryogenic distillation into its component parts, for example, nitrogen, oxygen and argon. Many applications involve the liquefaction of a gas in which the gas is compressed and then sufficiently cooled to produce a liquid.

Although in any compression application, it is possible to compress the gas in a single stage, it is more common to compress the gas in sequential compression stages. The reason for this is as the gas is compressed, its temperature rises and the elevated gas temperature requires an increase in power to compress the gas. Where the gas is compressed in stages, the gas may be cooled between stages to lower overall power requirements, as the process gets closer to isothermal compression than if no interstage cooling is done. In a typical compressor installation utilizing individual stages, each stage uses a centrifugal compressor in which gases entering an inlet to the compressor are distributed to a vaned compressor wheel that rotates to accelerate the gas and thereby impart the energy of rotation to the gas. This increase in energy is accompanied by an increase in velocity and a pressure rise. The pressure is recovered in a static vaned or vaneless diffuser that surrounds the compressor wheel and functions to decrease the velocity of the gas and thereby increase the gas pressure of the compressed gas.

The individual compressors of the compression stages can be driven by a common driver, such as an electric motor, through a transmission that consists of a single bull gear driving driven gears connected to the shafts of the compressors. The problem with such an arrangement is that an air separation plant does not always have the same demand placed upon it to deliver products. For example, in an air separation plant designed to produce oxygen, the demand can vary with the time of day and the day of the week. Since, the major cost in operating an air separation plant is electrical power, it is desirable to be able to turn down the plant and operate it to produce less oxygen than the plant is normally capable of producing during normal operating conditions. This can be done by reducing the flow of air into the multistage compression system used in compressing the air for the plant. The air flow can be reduced by reducing the speed of the compressors. Thus, in a geared arrangement, if the speed of the motor driving the compressors is decreased, the rotational speeds of all of the compressors will be reduced by the same amount. The problem with this is that the output pressure of the multistage compression system will also be reduced. Now, in case the oxygen or other gas is required to be produced at a particular pressure for a customer, such a reduction in pressure will also reduce the pressure of the product. As such, it is sometimes not possible to operate an air separation plant at turn down operating conditions. These same types of problems would occur in any type of operation involving the compression of a gas in stages.

Another problem with the common geared arrangement is that all of the compressors must be situated around the gearing. Further, there are inefficiencies in such a geared arrangement of compressors that arise from thermal losses from the gearing or gearbox. In order to avoid problems with having to mount compressors about a gear box or thermodynamic inefficiencies, the gearing or gearbox can be eliminated and the compressors can be individual driven and controlled with speed controllers. For example, US Patent Appln. No. 2007/0189905, discloses a multistage compression system that includes compressors connected in series with interstage cooling between the compressors. Each of the compressors is individually controlled by a speed controller. However, the speed of the compressors is controlled in this patent such that when the speed is increased or decreased, the ratio of the speed of any two motors remains the same. Consequently, there exist the same difficulties in such a system as would exist in a geared system used in an air separation plant. Although such a system can be turned down and operated in a turned down operating condition, the delivery pressure will be reduced.

As will be discussed, the present invention provides methods and apparatus that are particularly applicable to air separation in which a multistage compression system can be run under turn down operating conditions while continuing to deliver compressed gas at the pressure obtained during normal operational conditions.

SUMMARY OF THE INVENTION

The present invention provides a method of compressing a gas. In accordance with such method, the gas is compressed in a series of compression stages, from a lower pressure to a higher pressure. During normal operating conditions, the gas is supplied from the series of compression stages at the higher pressure and at a higher flow rate and during turndown conditions, the gas is supplied from the series of compression stages at the higher pressure and at a lower flow rate. In this regard, the term “flow rate” as used herein and in the claims means mass flow rate. The compression stages have compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors. In this regard, the term “variable speed motors” as used herein and in the claims means any type of device that can impart rotational motion to a compressor and in which the speed can be varied. Examples of variable speed motors include variable speed electric motors, variable speed hydraulic motors, variable speed internal combustion engines and variable speed stream turbines. During the turndown operating conditions, the speeds of the variable speed motors are adjusted and therefore, the speeds of the compressors such that an initial of the compressors associated with an initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors. Successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the gas to be delivered at the higher pressure.

When a multistage compression system is operated in accordance with the present invention, as set forth above, the flow rate is reduced during the turn down operating conditions while allowing the gas to be delivered at the pressure that is obtained during the normal operating conditions. Thus, the method of the present invention allows the multistage compression system to be used in applications where a stable pressure is required under all operational regimes. In addition to the foregoing, the method of the present invention is particularly energy efficient when the multistage compression system is operated during turn down conditions. The reason for this is that the power consumed by a multistage compression system have multiple stages is greatest at the initial compression stage because the volumetric flow rate is greatest in such stage due to the fact that the gas has the lowest density while being compressed in the initial stage. As the gas is successively compressed, the density increases and consequently less power is consumed in subsequent compression stage. However, since the initial compression stage is operated and turned down along its peak efficiency operating line, the power consumed by such stage is less than would otherwise have been consumed had the compression stages been turned down at a constant speed ratio. The successive stages will have to recover the reduction of the pressure in the first stage of compression due to turn down and as such, will very likely not operate along their peak efficiency operating line. However, since the successive stages will consume less power than the initial compression stage, the loss in efficiency will be more than compensated for through operation of the initial stage at its peak efficiency.

Although there are applications of multiple stage multistage compression systems in which the compressors operate adiabatically, in most applications the gas will be cooled between the compression stages and after having been compressed in the series of compression stages.

In applications of the present invention involving air separation, interstage and after cooling will invariably be used. In this regard, the present invention provides a method of separating air in which the air is compressed in a series of compression stages, with interstage cooling between the compression stages and after cooling to cool the air after having been compressed in the series of compression states. The air is compressed within the compression stages from a lower pressure to a higher pressure. During normal operating conditions, the air is supplied at the higher pressure to a main heat exchanger from the series of the compression stages at a higher flow rate of the air and during turndown conditions, the air is supplied air to a main heat exchanger at the higher pressure from the series of the compression stages and at a lower flow rate. The air is cooled, after having been compressed, within the main heat exchanger and is then introduced into a distillation column system to produce return and product streams. The return and product streams are warmed within the main heat exchanger to cool the air. The compression stages have compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors. During the turndown operating conditions, the speeds of the variable speed motors and therefore, the speeds of the compressors are adjusted such that an initial of the compressors associated with an initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio and below a design pressure ratio for the initial of the compressors. Successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the gas to be delivered at the higher pressure.

In a method of the present invention, during both normal operating conditions and turndown operating conditions, the gas can be supplied from a final compressor of a final of the compression stages. In an alternative embodiment of the present invention, during normal operating conditions, the gas can be supplied from a final compressor at the pressure and at the higher flow rate and with an auxiliary compressor by-passed. During the turndown operating conditions, when the intermediate and final compressor stages must operate beyond their efficient pressure ration conditions or beyond their mechanically acceptable speeds, the auxiliary compressor can be set in flow communication with the final compressor and the gas is then supplied from the auxiliary compressor at the pressure and at the lower flow rate. Additionally, in any method of the present invention, the variable speed motors can be direct drive motors. Speed controllers are connected to the direct drive motors to control the speed of each of the direct drive motors.

In another aspect, the present invention provides a multistage compression system for compressing a gas. In accordance with such aspect, a series of compression stages are provided to compress the gas from a lower pressure to a higher pressure in a final stage of the compression stages. The multistage compression system is configured to operate in a normal operating condition and in a turndown operating condition such that the air is supplied at the higher pressure from the compression stages and at a higher flow rate during the normal operating condition and the air is supplied at the higher pressure from the compression stages at a lower flow rate of the gas during the turndown condition.

The compression stages have compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors and variable speed controllers connected to the compressors and configured to independently adjust the speed of the compressors. A master controller is connected to the variable speed controllers and configured such that during the turndown operating conditions, the speeds of the variable speed motors and therefore, the speeds of the compressors are adjusted such that an initial of the compressors associated with the initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors. Successive compressors, located in successive compression stages, situated downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the compression stages to deliver the gas at the higher pressure.

As indicated above, intercoolers can be positioned between the compression stages. An aftercooler can be connected to the series of compression stages such that the gas is cooled after having been compressed in the series of compression stages.

The present invention also provides an air separation plant employing a series of compression stages to compress the air from a lower pressure to a higher pressure in a final stage of the compression stages. Intercoolers are positioned between the series of the compression stages and an aftercooler connected to the final stage of the compression stages. The multistage compression system is configured to operate in a normal operating condition and in a turndown operating condition such that the gas is supplied at the higher pressure from the compression stages and at a higher flow rate of the gas and the gas is supplied at the higher pressure from the compression stages at a lower flow rate of the gas during the turndown conditions. A main heat exchanger is connected to the multistage compression system and is configured to cool the air, after having been compressed. A distillation column system, configured to produce return and product streams, is connected to the main heat exchanger such that the air, after having been cooled in the main heat exchanger, is introduced into the distillation column system and the return and product streams warm within the main heat exchanger to cool the air. A master controller is connected to the variable speed controllers and configured such that during the turndown operating conditions, the speeds of the variable speed motors and therefore, the speeds of the compressors are adjusted such that an initial of the compressors associated with the initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors. Successive compressors, located in successive compression stages, situated downstream of the initial of the compression stages, operate at the lower flow rate and at increased speeds and higher pressure ratios that will enable the compression stages to deliver the gas at the higher pressure.

In a multistage compression system in accordance with the present invention or an air separation plant employing such a multistage compression system, the compression stages can be configured such that during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages. Alternatively, the compression stages can be provided with a final compressor in a final compression stage of the compression stages and an auxiliary compressor in an auxiliary compression stage of the compression stages. A flow control network is provided having a by-pass line, a first valve positioned between the by-pass line and an aftercooler connected to the final compressor. A second valve is positioned between the aftercooler and the auxiliary compressor. Each of the first valve and the second valves are operable to be set in a closed position and an open position such that during normal operating conditions, the first valve is set in the open position and the second valve is set in the closed position and the gas is supplied from a final compressor at the pressure and at the higher flow rate through the by-pass line and with the auxiliary compressor by-passed. During turndown operating conditions, the first valve is set in the closed position and the second valve is set in the open position such that the auxiliary compressor is connected to the aftercooler and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate. In any embodiment of the present invention, the variable speed motors can be direct drive motors.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention concludes with claims distinctly pointing out the subject matter in accordance with the present invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a multistage compression system for carrying out a method in accordance with the present invention;

FIG. 2 is a graphical representation of a map of compressor performance for the multistage compression system shown in FIG. 1 during normal operational conditions.

FIG. 3 is a graphical representation of a map of compressor performance for the multistage compression system shown in FIG. 1 during turndown operational conditions.

FIG. 4 is an alternative embodiment of FIG. 1; and

FIG. 5 is a schematic diagram of an air separation plant incorporating a multistage compression system shown in FIG. 1 or FIG. 4. for carrying out a method in accordance with the present invention.

DETAILED DESCRIPTION

With reference to FIG. 1, a multistage compression system 1 in accordance with the present invention is illustrated. Multistage compression system 1 is a multistage compression unit having four compression stages 10, 12, 14 and 16 that is designed to compress a gas contained in a feed stream 18 from a lower pressure to a higher pressure and thereby produce a compressed gas stream 20 containing the gas at the higher pressure. It is understood that a multistage compression system in accordance with the present invention could include a greater or lesser number of stages.

Compression stage 10 is provided with a compressor 22 that is driven by a variable speed motor 24. It is understood that the compressor 22 can be centrifugal compressor of the type described above that is driven by a variable speed electric permanent magnet motor. In compression stage 10, the variable speed motor 24 is controlled by a speed controller 26 that could be a variable frequency controller where the driver or motor (now-a-days, “drive” typically refers to transmissions or motor controllers. Could be misleading here.) 24 is a variable speed electric permanent magnet motor. It is to be pointed out that the variable speed motor 24 or any other motor employed in connection with the present invention could be another type of device, for example, a throttle controlled steam turbine. The successive compression stages 12, 14 and 16 are similarly provided with compressors 28, 30 and 32, variable speed motors 34, 36 and 38 and speed controllers 40, 42 and 44, respectively.

Multistage compression system 1 also incorporates interstage cooling and after cooling. As known in the art, as the gas is compressed in each stage of compression, the temperature of the gas rises. As a result, the density of the gas decreases and more power has to be expended in a stage to compress the gas. By cooling the gas between stages, the density of the gas is greater than without such cooling resulting in a power savings. This being said, it is possible to have a multistage compressor installation without intercooling or after cooling where it is desired to produce a heated gas that can be used in a downstream process. In the illustrated embodiment, however, such intercooling is illustrated and for such purposes, after the gas is compressed in compressor 22, it is cooled in interstage cooler 46 before being fed to the inlet of the downstream compressor 28. Similarly, after the gas has been further compressed in compressor 28, it is cooled in interstage cooler 48 before being fed into the inlet of compressor 30 and after the gas has been compressed in compressor 30, the gas is cooled in interstage cooler 50. After the gas has been compressed in compressor 32, it is cooled in aftercooler 52 to remove the heat of compression. Interstage coolers 46, 48 and 50 are known in the art and can consist of liquid cooled, fin and tube heat exchangers. Aftercooler 52 is preferably also incorporates liquid cooled, fin and tube heat exchanger construction or liquid cooled, direct contact heat exchange.

Multistage compressor system 1 is designed to produce compressed gas stream 20 at a specific pressure and flow rate. When multistage compressor system 1 is operating in this manner, it is operating under design conditions as are the compression stages 10, 12, 14 and 16 and their associated compressors 22, 28, and 32, respectively. When it is desired to reduce the flow of compressed gas stream 20, multistage compressor system 1 will still deliver the compressed gas stream at the same specific pressure that is required under design operating conditions, but at the reduced flow. Under such circumstances, multistage compressor system 1 is said to be operating under turndown operating conditions and therefore, the compression stages 10, 12, 14 and 16 and their associated compressors 22, 28, 30 and 32. The control is exercised by a programmable logic controller 52 that sends appropriate control signals through a control network 54 that can be a set of data transmission lines to variable speed controllers 26, 40, 42 and 44 that act to control the speed of motors 24, 34, 36 and 38 and therefore, the speed of the compressors 22, 28, 30 and 32.

With reference to FIG. 2, a normalized map of compressor performance is illustrated in which under the design operating conditions all of the compressors 22, 28, 30 and 32 are operating along their peak efficiency operating line where the pressure ratio is proportional to the flow through the compressors and therefore, at peak efficiency and at 100% of their speed at the design operating condition. In accordance with the present invention, where it is desired to reduce the flow under turn down operating conditions, the speed of the compressor 22 is downwardly adjusted along the peak efficiency operating line. This will reduce the pressure ratio across the compressor 22 and this reduction will have to be made up in the successive compressors 28, 30 and 32 of the successive compression stages. One way to do this is to compute the compression ratio between the turned down initial compression stage 10 and divide the computed pressure ratio equally among the successive compression stages 12, 14 and 16 (there may be more optimal methods than to divide the computed ratio evenly over the remaining stages). In the illustrated embodiment, the computation is performed in the programmable logic computer 52 which also, generates the required control signals that are applied to the variable speed controllers through the control network 54. While the subsequent compressors 28, 30 and 32 will no longer be operating along the peak efficiency operating line, the compressor 22 of the initial compression stage 10 will remain operating along the peak efficiency operating line and therefore, be operated efficiently. Why this is important is that the compressor 22 will invariably have the greatest electrical power requirements. Thus, during turndown operating conditions, the multistage compressor system will invariably be more efficient than if the compressor speeds were stepped down together as provided for in the prior art.

With additional reference to FIG. 3 and the Table below, an example of the operation of multistage compression system 1 is illustrated that is particularly applicable to a cryogenic air separation plant of the type that is illustrated in FIG. 4 to be discussed below.

TABLE Inlet Pressure, psia 15 Train Discharge Pressure, psia 90 Stage Count 4.0 Train Pressure Ratio 6.00 Design Stagewise Pressure Ratios 1.565 Turndown Rate 80% 1^(st) Stage Peak Efficiency Pressure 70% Ratio at Turndown Pin Pout Volumetric Pressure Normalized psia Flow Ratio PR Flow Full Stage 1 15.0 23.5 1.00 1.57 1.000 1.000 Flow Stage 2 23.5 36.7 0.64 1.57 1.000 1.000 Case Stage 3 36.7 57.5 0.41 1.57 1.000 1.000 Stage 4 57.5 90.0 0.26 1.57 1.000 1.000 80% Stage 1 15.0 16.4 0.80 1.10 0.700 0.800 Flow Stage 2 16.4 29.0 0.91 1.76 1.126 1.143 Case Stage 3 29.0 51.1 0.52 1.76 1.126 1.015 Stage 4 51.1 90.0 0.29 1.76 1.126 0.901 As shown in the Table, the inlet pressure of the feed stream 18 is about 15 psia and the required discharge pressure is 90 psia of the compressed gas stream 20 for a total pressure ratio of 6.00 across the multistage compressor system 1. Each of the compressors 22, 28, 30 and 32 operate at a design pressure ratio of 1.57. As illustrated the flow through successive compressors decreases because the volumetric flow is decreasing as the density of the gas decreases due to the compression of the gas. During turndown operating conditions of 80 percent of the flow, the normalized pressure ratio across compressor 22 will be adjusted along the peak efficiency operating line to be 0.7. If the required pressure ratio is equally adjusted among the successive stages 12, 14 and 16, then each compressor 28, 30 and 32 will be operated at a normalized pressure ratio of 1.126 in accordance with the power law. In other words, 0.700×1.126×1.126×1.126 is equal to 1.00 or taking the actual pressure ratios, 1.10×1.76×1.76×1.76 is equal to 6.00. Therefore, although multistage compression system 1 is being turned down to 80 percent of flow, it will still deliver the compressed gas stream 20 at 90 psia or in other words, at the design pressure. To achieve these conditions, the speed of the compressors 22, 28, 30 and 32 will be adjusted by their respective variable speed controllers 26, 40, 42 and 44 to speeds indicated by FIG. 3: compressor 22 will be operated at approximately 83 percent of its design rotational speed; compressor 28 will be operated at approximately 107 percent of its design rotational speed; compressor 30 will be operated at approximately 104 percent of its design rotational speed; and compressor 32 will be operated at approximately 103 percent of its design rotational speed. Programmable logic computer 54 could be programmed to simply have a set of pre-set turn down conditions or computations could be made as set forth above to turn the multistage compression system 1 down to a set turn down flow rate that would be an input to programmable logic computer 1.

Additionally, as illustrated, there exists pressure transducers 58, 60, 62 and 64 to measure the pressure ratios across the compressors 28, 30 and 32. Signals referable to the pressures could then serve as a further input by means of data transmission lines 66, 68, 70 and 72, respectively, to the programmable logic controller 54. Also, a flow meter 74 able to send a signal via a data transmission line 76 to the programmable logic controller 54 along with a pressure transducer 78 and an associated data transmission line 80 could also be present.

This instrumentation can and the programmable logic controller 54 may be used to implement the control operation of the multistage compression system 1 described above. To such end, the programmable logic controller 54 can be programmed to receive an input data referable to the desired flow rate and then to compute a reduced speed for variable speed motor 24 in accordance with an algebraic representation of FIG. 3 or a database containing the data within FIG. 3 and generate a control signal referable to such computed reduced speed. The control signal is then input into the variable speed controller 26 through data transmission line 56 that in turn controls the speed of variable speed motor 24 to obtain the reduced speed. Flow meter 74 generates a signal referable to the mass flow rate of compressed gas stream 20 that is transmitted as an input into programmable logic controller 54 by means of data transmission line 80. Programmable logic controller is programmed to respond to such signal and send a revised signal to variable speed controller 26 should the flow rate not be within 2 percent of the desired flow rate input into programmable logic controller 54. When the flow rate has been set, the further computations are performed by programmable logic controller 54 to compute the pressure ratios in a manner set forth in the above example, generate control signals to be transmitted by data transmission line 56 to variable speed controllers 40, 42 and 44 to in turn adjust the speeds of variable speed motors 34, 36 and 38 and therefore, compressors 28, and 32 to achieve the desired pressure ratios computed by programmable logic controller 54. Pressure signals referable to inlet and outlet pressures are generated by pressure transducers 58, 60, 62 and 64 and fed as another input into programmable logic computer 54 that computes the actual pressure ratios and then as necessary generates and individually updates the control signals being sent to variable speed controllers 44, 42 and 44 until measured pressure ratios are within 2 percent of the computed pressure ratios. Increased thereafter, data generated on the basis of the signal referable to mass flow as that is produced by flow meter 74 is used to confirm that the desired train flow is achieved. If the desired flow rate is not achieved, the process outlined above is repeated until the flow rate is within about two percent of the desired flow rate.

For purpose of a simplification, FIGS. 2 and 3 and the above example assume aerodynamically identical compressor stages; that is, while physically different and operating at differing pressure and volumetric flows, each stage conforms to identical design rules and operates such that their fluid dynamic flows are effectively scaled variants of one another.

However, operation of multistage compression system 1 is not limited to using aerodynamically identical compressor stages. Non-aerodynamically identical stages may be used as well. This requires programmable logic controller 54 to use for example, algebraic representations of each stages unique flow, pressure and speed behavior.

With reference to FIG. 4, a multistage compression system 1′ is illustrated. For the sake of simplicity of explanation, where an element illustrated in this FIG. 4 has been described in FIG. 1, the same reference numbers will be used. In the multistage compression system 1′, a final compression stage 14′ is illustrated having a final compressor 30′ and an auxiliary compressor 16′ in an auxiliary compressions stage 16′. Also included is a flow control network 90. The flow control network 90 is provided with a by-pass line 92, a first valve 94 positioned within the by-pass line 92 and a second valve 96 positioned between the aftercooler 50 and the auxiliary compressor 16′. Each of the first valve 94 and the second valve 96 are operable to be set in a closed position and an open position and to be activated by the programmable logic controller 54′ through electrical connections 98 and 100, respectively. The programmable logic controller is programmed such that during normal operating conditions, the first valve 94 is set in the open position and the second valve 96 is set in the closed position and the compressed gas stream 20′ is thereby supplied from the final compressor 30′ at the design pressure and at the higher flow rate through the by-pass line 92. Under such circumstances, the auxiliary compressor 32′ is by-passed. During turndown operating conditions, the first valve 94 is set in the closed position and the second valve 96 is set in the open position such that the auxiliary compressor 30′ supplied compressed gas through the aftercooler 50 and to the auxiliary compressor 32′. The auxiliary compressor 32′ thereby supplies the compressed gas stream 20′ from its associated aftercooler 52 at the same design pressure, but at the lower flow rate to be obtained during turndown operation conditions. In this regard, the multistage compression system 1′ is turned down in a similar fashion to the multistage compression system 1 in that the speed of the initial compressor 22 of the compression stage 10 is downwardly adjusted along the peak efficiency operating line and the required pressure ratio for the subsequent compressors 28, 30′ and 16′ are divided to deliver the compressed gas stream 20′ at the required design delivery pressure in a manner set forth above. The compressor system 1′ permits the delivery of reduced flow at design pressure while turning down compressor 30′ and its preceding stages each along its peak efficiency operating line.

With reference to FIG. 5, an air separation plant 2 is illustrated that incorporates a multistage compression system 110 that can either be in the form illustrated in connection with FIG. 1 or 4 and controlled in a manner that has been described above. The compression system 110 compresses a feed air stream 112 to produce a compressed air stream designated by reference numeral 114 that could therefore, be the compressed stream 20 or 20′ produced by multistage compression systems 1 and l′, respectively. It is also to be pointed out that air separation plant 2 does not include a pre-purification unit to remove higher boiling impurities from the feed air stream 112 in that the air separation plant 2 is designed to be used in an enclave of air separation plants as such, the feed air could be centrally processed. By the same token, if air separation plant 2 were to be used as a stand-alone plant, a pre-purification unit could be located downstream of the multistage compression system 110. Compressed air stream 114 is divided into a main compressed air stream 116 and two subsidiary compressed air streams 118 and 120. Main compressed air stream 116 is cooled within a main heat exchanger 122 to a temperature suitable for its distillation in an air separation unit 124.

Subsidiary compressed air stream 118 is compressed in a booster compressor 126 to produce a boosted pressure air stream 128 that is cooled in an after-cooler 130 and then partially cooled within main heat exchanger 122 to an intermediate temperature between the cold and warm end temperature of main heat exchanger 122. The boosted pressure air stream 128, after having been partially cooled, is introduced into a turbo expander 138 coupled to the booster compressor 126 to produce an exhaust stream 140 that is reintroduced into main heat exchanger 122 and then fully cooled and introduced into the air separation unit 124. The purpose of the foregoing is to impart refrigeration into the air separation plant 2 for purposes of overcoming warm end heat exchanger losses, heat leakage into the cold box housing the air separation unit 124 and to create liquids. However, it is understood that there are air separation plants in which refrigeration is externally applied and also, air separation plants in enclaves in which the refrigeration is centrally applied. Also, there are yet other methods of using turbo expanders to supply refrigeration to air separation plants as are well known in the art. Consequently, the foregoing means used to impart refrigeration in air separation plant 2 is shown for purposes of illustration only and is not intended to limit the scope of the present invention.

Subsidiary compressed air stream 120 is in turn introduced into booster compressor 142 to produce a boosted pressure air stream 144 that after cooling in aftercooler 144 is fully cooled in main heat exchanger 122 to produce a liquid air stream 146 that is also introduced into air separation unit 124. The production of boosted pressure air stream 144 is necessary to warm a pumped return stream 152 that could be liquid oxygen or liquid nitrogen and is shown for purposes of illustration only.

Air separation unit 124, as known in the art, could be a double column unit having a high pressure column and a low pressure column operatively associated within one another in a heat transfer relationship by a condenser reboiler. Both of such columns have mass transfer contacting elements such as trays, structured packing, random packing or a combination of such elements. These element contact liquid and vapor phases of the air in a manner known in the art such that the liquid phases becomes ever more rich in oxygen as it descends and the vapor phase becomes ever more rich in nitrogen as its ascends in either the high or low pressure columns. As known in the art, the high pressure column produces a crude liquid oxygen column bottoms that is further refined in the low pressure column and a nitrogen-rich vapor column overhead that is condensed in the condenser reboiler to produce reflux for both the high and low pressure columns. The low pressure columns produces an oxygen-rich liquid that can be removed as return stream 148, pumped in a pump 150 to produce the pumped return stream 150 which is fully warmed within main heat exchanger 122 to produce a product at pressure. The low pressure column also produces a nitrogen-rich vapor column overhead that could be removed as a return stream 154 and then, fully warmed in main heat exchanger 122 to produce a nitrogen product.

Although not illustrated, air separation unit 124 would incorporate other known heat exchangers such as a subcooling unit to subcool the reflux to the low pressure column and the crude liquid oxygen to be further refined in the low pressure column and expansion valves to expand such streams to a suitable pressure for introduction into the low pressure column. In the illustrated embodiment, liquid air stream 146 could be expanded by expansion valves and introduced into the low pressure column and also the high pressure column. The exhaust stream 140 could be introduced into the low pressure column or possibly the high pressure column. The main heat exchanger is typically a brazed aluminum unit or units arranged in parallel. Further the main heat exchanger could also incorporate a high pressure unit designed to cool the boosted pressure air stream 142 and to warm the pumped return stream 152. Another known possibility is that the air separation unit 124 could incorporate an argon column or columns to produce an argon product. Also, air separation unit 124 could be a single column designed to produce a nitrogen product.

As indicated above, multistage compression unit 110 is designed to function to produce the compressed air stream 114 at a constant pressure both during normal operational conditions of air separation plant 2 where air separation plant 2 is making products, such as pumped return stream 152, at a design production rate and during turn down operating conditions where products are produced at a lower flow rate. During turndown operation, the multistage compression system 110 is turned down to reduce the flow rate of compressed air stream 114. Compressed air stream 114 is divided into a main compressed air stream 116 and two subsidiary compressed air streams 118 and 120. The reduced flow in compressed air stream 114 may be distributed among: subsidiary compressed air stream 120 thereby reducing the production of liquid air stream 146; subsidiary compressed air stream 118 thus reducing the refrigeration available to air separation plant 2; and/or reducing the flow rate of compressed air stream 116 so that production is decreased. Distribution of the reduced flow from compressed air steam 114 may be apportioned in any way among compressed air streams 116, 118, 120 such that the sum of flows in compressed air steams 116, 118 and 120 totals the flow in compressed of stream 114. As an example in a plant having high and low pressure columns that is designed to produce a pressurized oxygen product, the pumped return stream 152 would be made up of oxygen-enriched liquid removed from the low pressure column. If it were desired during turn down operating conditions to reduce the production of the pressurized oxygen product, the flow of streams 116 and 120 would be reduced and the flow rate of stream 118 could remain the same as during design operation. The speed of the booster compressor 142 would have to be increased to allow the discharge pressure to remain at design.

Although the present invention has been described with respect to preferred embodiments, as will occur to those skilled in the art, numerous changes, additions and omission can be made without departing from the spirit and scope of the present invention as set forth in the appended claims. 

1. A method of compressing a gas comprising: compressing the gas in a series of compression stages from a lower pressure to a higher pressure; during normal operating conditions, supplying the gas from the series of compression stages at the higher pressure and at a higher flow rate and during turndown conditions, supplying the gas from the series of compression stages at the higher pressure and at a lower flow rate; the compression stages having compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors; and during the turndown operating conditions, adjusting the speeds of the variable speed motors and therefore, the speeds of the compressors such that an initial of the compressors associated with an initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the gas to be delivered at the higher pressure.
 2. The method of claim 1, wherein during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages.
 3. The method of claim 1, wherein: during normal operating conditions, the gas is supplied from a final compressor at the pressure and at the higher flow rate and with an auxiliary compressor by-passed; and during turndown operating conditions, the auxiliary compressor is in flow communication with the final compressor and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.
 4. The method of claim 1, wherein the gas is cooled between the compression stages and after having been compressed in the series of compression stages.
 5. The method of claim 1, wherein the variable speed motors are direct drive motors and speed controllers are connected to the direct drive motors to control the speed of each of the direct drive motors.
 6. The method of claim 4, wherein the variable speed motors are direct drive motors and speed controllers are connected to the direct drive motors to control the speed of each of the direct drive motors.
 7. A method of separating air comprising: compressing the air in a series of compression stages, with interstage cooling between the compression stages and after cooling to cool the air after having been compressed in the series of compression states, the air compressed within the compression stages from a lower pressure to a higher pressure; during normal operating conditions, supplying the air at the higher pressure to a main heat exchanger from the series of the compression stages at a higher flow rate of the air and during turndown conditions, supplying the air to a main heat exchanger at the higher pressure from the series of the compression stages and at a lower flow rate; cooling the air, after having been compressed, within the main heat exchanger and introducing the air into a distillation column system to produce return and product streams; warming the return and product streams within the main heat exchanger to cool the air; the compression stages having compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors; and during the turndown operating conditions, adjusting the speeds of the variable speed motors and therefore, the speeds of the compressors such that an initial of the compressors associated with an initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the gas to be delivered at the higher pressure.
 8. The method of claim 7, wherein during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages.
 9. The method of claim 7, wherein: during normal operating conditions, the gas is supplied from a final compressor at the pressure and at the higher flow rate and with an auxiliary compressor by-passed; and during turndown operating conditions, the auxiliary compressor is in flow communication with the final compressor and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.
 10. The method of claim 7, wherein the variable speed motors are direct drive motors and speed controllers are connected to the direct drive motors to control the speed of each of the direct drive motors.
 11. A multistage compression system for compressing a gas comprising: a series of compression stages to compress the gas from a lower pressure to a higher pressure in a final stage of the compression stages; the multistage compression system configured to operate in a normal operating condition and in a turndown operating condition such that the air is supplied at the higher pressure from the compression stages and at a higher flow rate during the normal operating condition and the air is supplied at the higher pressure from the compression stages at a lower flow rate of the gas during the turndown condition; the compression stages having compressors driven by variable speed motors capable of driving the compressors at speeds that are able to be independently adjusted for each of the compressors, thereby to adjust flow rate through the compressors and pressure ratios across the compressors and variable speed controllers connected to the compressors and configured to independently adjust the speed of the compressors; and a master controller connected to the variable speed controllers and configured such that during the turndown operating conditions, the speeds of the variable speed motors and therefore, the speeds of the compressors are adjusted such that an initial of the compressors associated with the initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and in successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the compression stages to deliver the gas at the higher pressure.
 12. The multistage compression system of claim 11, wherein the compression stages are configured such that during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages.
 13. The multistage compression system of claim 11, wherein: the compression stages have a final compressor in a final compression stage of the compression stages and an auxiliary compressor in an auxiliary compression stage of the compression stages; a flow control network having a by-pass line, a first valve positioned between the by-pass line and an aftercooler connected to the final compressor and a second valve positioned between the aftercooler and the auxiliary compressor; each of the first valve and the second valves operable to be set in a closed position and an open position such that during normal operating conditions, the first valve is set in the open position and the second valve is set in the closed position and the gas is supplied from a final compressor at the pressure and at the higher flow rate through the by-pass line and with the auxiliary compressor by-passed; and during turndown operating conditions, the first valve is set in the closed position and the second valve is set in the open position such that the auxiliary compressor is connected to the aftercooler and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.
 14. The multistage compression system of claim 11, wherein intercoolers are positioned between the compression stages and an aftercooler is connected to the series of compression stages such that the gas is cooled after having been compressed in the series of compression stages.
 15. The multistage compression system of claim 11, wherein the variable speed motors are direct drive motors.
 16. The multistage compression system of claim 14, wherein the variable speed motors are direct drive motors.
 17. An air separation plant comprising: a series of compression stages to compress the air from a lower pressure to a higher pressure in a final stage of the compression stages; intercoolers between the series of the compression stages and an aftercooler connected to the final stage of the compression stages; the multistage compression system configured to operate in a normal operating condition and in a turndown operating condition such that the gas is supplied at the higher pressure from the compression stages and at a higher flow rate of the gas and the gas is supplied at the higher pressure from the compression stages at a lower flow rate of the gas during the turndown conditions; a main heat exchanger connected to the multistage compression system and configured to cool the air, after having been compressed; a distillation column system configured to produce return and product streams, the distillation column system connected to the main heat exchanger such that the air, after having been cooled in the main heat exchanger, is introduced into the distillation column system and the return and product streams warm within the main heat exchanger to cool the air; and a master controller connected to the variable speed controllers and configured such that during the turndown operating conditions, the speeds of the variable speed motors and therefore, the speeds of the compressors are adjusted such that an initial of the compressors associated with the initial of the compression stages operates along a peak efficiency operating line for the initial of the compressors at which the pressure ratio is directly proportional to the flow rate and such that the lower flow rate is obtained at a reduced pressure ratio, below a design pressure ratio for the initial of the compressors and in successive compressors, located in successive compression stages, downstream of the initial of the compression stages, operate at the lower flow rate and at pressure ratios that will enable the compression stages to deliver the gas at the higher pressure.
 18. The air separation plant of claim 17, wherein the compression stages are configured such that during both normal operating conditions and turndown operating conditions the gas is supplied from a final compressor of a final of the compression stages.
 19. The air separation plant of claim 17, wherein: the compression stages have a final compressor in a final compression stage of the compression stages and an auxiliary compressor in an auxiliary compression stage of the compression stages; a flow control network having a by-pass line, a first valve positioned between the by-pass line and an aftercooler connected to the final compressor and a second valve positioned between the aftercooler and the auxiliary compressor; each of the first valve and the second valves operable to be set in a closed position and an open position such that during normal operating conditions, the first valve is set in the open position and the second valve is set in the closed position and the gas is supplied from a final compressor at the pressure and at the higher flow rate through the by-pass line and with the auxiliary compressor by-passed; and during turndown operating conditions, the first valve is set in the closed position and the second valve is set in the open position such that the auxiliary compressor is connected to the aftercooler and the gas is supplied from the auxiliary compressor at the pressure and at the lower flow rate.
 20. The air separation plant of claim 17, wherein the variable speed motors are direct drive motors. 